For the past seven years a microspectrophotometry laboratory devoted to the study of visual pigments in single vertebrate photoreceptors has been in full operation. It is built around a unique photon-counting microspectrophotometer (PMSP) designed to measure very small optical densities in areas small as one square micrometer with the highest achievable signal to noise ratio. Because of the difficulty of extracting and purifying cone pigments, we are concentrating on studying them in situ. This also permits the identification of particular pigments with receptor morphology and provides information on the location and numbers of different receptor types in the retinal mosiac. Our aims are: 1. To obtain precise information on the pigment absorption of the receptors of each spectral class then to average the results of large numbers of similar receptors to determine the spread of the maximum values, and to determine the existence of spectral subclasses accurately, then to characterize these and any receptors containing anomolous pigments. 2. To study the arrangements of the receptors in the retinal mosaic. 3. To study the organelles of the inner segments such as oil droplets, ellipsosomes, and mitochondria to determine how they modify spectral sensitivity, and to investigate the possibility that they are involved in damage to the retina by light of short wavelengths. 4. To maintain and improve the equipment and techniques u sed in retinal microspectrophotometry and to make the facility available for qualified investigators from other institutions. Indeed most of the work we have done to date has been accomplished with the aid of outside collaborators. Significance: a) To obtain a complete understanding of how the visual system processes color information, it is necessary to have as a starting point an accurate knowledge of the spectral absorptions of the receptors, then to compare these with the electrical responses to colored lights further along the visual pathways and with behavorial data. b) In prolonging the time available for studying receptors we use a variety of solutions and techniques which successfully retard receptor degeneration. These studies may increase our knowledge of mechanisms of degenerative retinal diseases. c) Although most of our work has been on fishes, turtles and amphibia, we are now obtaining a pair of macaque monkey eyes a week for study. Because of the great similarity of the macaque visual system to that of man we believe our results will be applicable to human vision and retinal disease.